Chert / Chalcedony

Physical Properties

Hardness: 7 (Mohs scale); Color: Tan, beige, brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich fluids in limestone or volcanic ash deposits. It is often created during diagenesis as silica replaces organic material or voids in sedimentary rocks.

Uses & Applications

Historically used for stone tools (arrowheads). Modern uses include abrasives, decorative stone, and as a component in aggregate for construction. Collectors value unique color patterns.

Geological Facts

Chert was one of the most important minerals for early humans because its conchoidal fracture creates edges sharper than a steel razor. It is virtually chemically inert.

Field Identification & Locations

Identify by its 'waxy' appearance, extreme hardness (it will scratch glass), and shell-like (conchoidal) curved breaks. Commonly found in stream beds or weathered out of limestone cliffs.

Identified on: 4/16/2026

Mode: Standard